[U-Boot] How does Driver Model UART work?

[Masahiro YAMADA]

Hi Simon,



2014-10-03 22:51 GMT+09:00 Simon Glass <sjg at chromium.org>:
> Hi Masahiro,
>
> On 3 October 2014 07:04, Masahiro Yamada <yamada.m at jp.panasonic.com> wrote:
>> Simon,
>>
>>
>>
>> I am totally being confused.
>>
>>
>>
>> As far as I looked at the dm code,
>> the private data is calloc'ed in device_probe() function
>>
>>         if (drv->priv_auto_alloc_size) {
>>                 dev->priv = calloc(1, drv->priv_auto_alloc_size);
>>                 if (!dev->priv) {
>>                         ret = -ENOMEM;
>>                         goto fail;
>>                 }
>>         }
>>
>>
>>
>> So,  dev->priv is storing the address of the allocated memory.
>>
>> Am I understanding correctly?
>
> Yes, it always does in driver model. This driver is no different.
>
> BTW the ns16550 driver is probably the oddest in U-Boot - it looks
> like UniPhier has its own UART driver so it would be better to convert
> that I think. See below for ideas on other UART drivers to look at
> which are much more normal.
>
>>
>>
>>
>> If so, I can't understand the following code:
>>
>>
>> static int ns16550_serial_getc(struct udevice *dev)
>> {
>>         struct NS16550 *const com_port = dev_get_priv(dev);
>>
>>         if (!serial_in(&com_port->lsr) & UART_LSR_DR)
>>                 return -EAGAIN;
>>
>>
>>
>> "com_port" is dev->priv, so it is pointing to the allocated area on RAM,
>> I guess.
>>
>>
>> It looks like
>> serial_in(&com_port->lsr)  is trying to read from the hardware register ?
>>
>> Or reading from malloc area RAM ??
>
> If you go one level deeper you will see that serial_in() is defined at
> the top in about 5 ways, one of which is used for driver model:
>
> #define serial_in(addr) \
>      ns16550_readb(com_port, addr - (unsigned char *)com_port)
>
> So it used com_port which is not an parameter. The parameter addr is
> only used to specify the register. This is actually the same as the
> non-DM code except in that case the parameter specifies the register
> and hardware address at the same time.

Without your help, I never could have understood this code.
Now it is clearer, thanks! (although this code is really unfortunate...)


> This is done since the internal NS16550_t type is unfortunately
> exported all over U-Boot. This is annoying because it is actually an
> internal register format for the UART. Even worse is the fact that the
> structure changes depending on CONFIG_SYS_NS16550_REG_SIZE. We can't
> have this sort of thing in driver model - we need to be able to cope
> with the device tree specifying all the information that the UART
> needs. So for driver model:

Agreed.
We should pass "reg-shift" from a device tree or a board-file.

If all the board switched to the driver model,
the driver code would become much clean.

>
> /*
>  * For driver model we always use one byte per register, and sort out the
>  * differences in the driver
>  */
> #define CONFIG_SYS_NS16550_REG_SIZE (-1)
>
> (see ns16550.h header for where this is used)
>
> I would like to avoid having that type exported and use a different
> method to pass the UART information around. But it is used in about 30
> places . So this approach allows us to move forward bit by bit,
> without duplicating the UART driver and creating a maintenance
> headache.
>
> In terns of implementing a new UART driver, are you using device tree?

UniPhier SoCs have not supported the device tree control yet.

I am planning to do the dm conversion with a board file.


> If so then you should just be able to follow along with Tegra - it's
> really easy - just copy that serial_tegra.c and adjust the device tree
> compat string and input clock. Other examples are serial_omap and
> serial_dw. (See u-boot-dm/working)

These are unfortunate.
They are borrowing ns16550 code.


> If you are using platform data, then the examples are serial_mxc and
> serial_pl01x.

I am doing this way.

Anyway, the register map of UniPhier is not 8250-compatible.
It cannot borrow ns16550 code.


-- 
Best Regards
Masahiro Yamada